1. Field of the Invention
Example embodiments relate generally to a pixel circuit, an organic light emitting display device, and a method of driving a pixel circuit. More particularly, embodiments of the inventive concept relate to a pixel circuit that compensates a threshold voltage distribution of a transistor using a few transistors and capacitors, an organic light emitting is display device that includes the pixel circuit, and a method of driving the pixel circuit.
2. Description of the Related Art
Recently, various flat panel display devices that reduce weight and volume (i.e., weakness of Cathode Ray Tube (CRT)) have been developed. The flat panel display devices include a liquid crystal display (LCD) device, a field emission display (FED) device, a plasma display panel (PDP), an organic light emitting display (OLED) device, etc.
The OLED device has advantages such as rapid response speed and low power consumption because the OLED device among the flat panel display devices displays an image using an organic light emitting diode that emits light based on recombination of electrons and holes.
Usually, the OLED device may be divided into a passive matrix type organic light emitting display (PMOLED) device and an active matrix type organic light emitting display (AMOLED) device according to a method of driving organic light emitting elements.
The AMOLED device has a plurality of scan-lines, a plurality of data-lines, a plurality of power source voltage lines, and a plurality of pixel circuits. The pixel circuits are arranged in a matrix form while being coupled to the lines. In addition, each of the pixel circuits usually includes an organic light emitting element, two transistors (i.e., a switching transistor for transferring a data signal and a driving transistor for driving the organic light emitting element based on the data signal), and one capacitor for maintaining the data signal.
As described above, although the AMOLED device has low power consumption, current intensity varies by deviation of the threshold voltage of the driving transistor (i.e., deviation of a difference between a gate voltage and a source voltage of the driving transistor). As a result, a variation of the current intensity results in display unevenness.
Namely, it is difficult to manufacture transistors of the AMOLED device to have the same characteristics (i.e., the deviation of the threshold voltage occurs among pixel circuits) because characteristics of the transistors included in the pixel circuits are changed according to manufacturing process variations.
Thus, recently, a compensation circuit including transistors and capacitors is suggested to overcome the problems. In addition, the compensation circuit is added into each of the pixel circuits. In this case, however, each pixel needs to include many transistors and capacitors therein.
In detail, as a compensation circuit is added into each of the pixel circuits, transistors, capacitors, and signal-lines controlling the transistors are added. Thus, in case of AMOLED device employing a bottom emission technique, the aperture ratio decreases and the probability of defects increases because the circuit has more components and a structure of the circuit is more complicated.
Further, recently, more than 120 Hz high-speed scan driving is required to remove motion blur phenomenon. In this case, however, recharge time decreases drastically for each of scan-lines. Namely, a capacitive load increases when each of the pixel circuits has the compensation circuit, and many transistors are formed in each of the pixel circuits coupled to a scan-line. As a result, it is difficult to implement the high-speed scan driving.